1. Field of the Invention
The present invention relates to a multilevel voltage generating circuit, a data driver using it, and a liquid crystal display apparatus with the data driver, and more specifically, to a layout pattern of a resistor string of the multilevel voltage generating circuit. The Japanese Patent Application Nos. 2006-298551 and 2007-281525 also relate to a multilevel voltage generating circuit, a data driver using it, and a liquid crystal display apparatus with the data driver. The disclosures of the Japanese Patent Application Nos. 2006-298551 and 2007-281525 are incorporated herein by reference.
2. Description of the Related Art
A resistor string has a plurality of resistors connected with one another through a plurality of division electrodes, divides reference voltages, and outputs a plurality of divided voltages (level voltages) from the plurality of division electrodes. As one example of the resistor string, a resistor string described in Japanese Laid Open Patent Application (JP-A-Heisei 8-213912: related art 1) will be described. In the resistor string described in the related art 1, a single resistance element is provided with contacts and electrodes both arranged in a same interval, each of which outputs a divided voltage. FIG. 1 is a plan view showing a layout pattern of the resistor string described in the related art 1.
Referring to FIG. 1, a resistor string 50 according to the related art 1 will be described. A resistor string 50 is a single resistance element provided with (N+1) contacts 54-0 to 54-N in a same interval. The resistor string 50 divides a voltage difference between reference voltages VG0 and VGN supplied to the contacts 54-0 and 54-N, and outputs the divided voltages V0 to VN through the contacts 54-0 to 54-N. Specifically, a wiring 51 to which the reference voltage VG0 is supplied is connected to the contact 54-0, and a wiring 52 to which the reference voltage VGN is supplied is connected to the contact 54-N. Wirings 53-1 to 53-(N-1) are connected to respective contacts 54-1 to 54-(N-1). In such a configuration, the voltage difference between the two reference voltages VG0 and VGN is divided by resistors R between these contacts (between dividing electrodes), and the voltages thus obtained are supplied to the nodes 56-1 to 56-(N-1) as the divided voltages V1 to VN−1. Moreover, the reference voltages VG0 and VGN are supplied to the nodes 56-0 and 56-N as the divided voltages V0 and VN through the wirings 51 and 52, respectively.
In recent years, high-accuracy voltage division is demanded, and a technique of improving an accuracy of division resistors is required. For this reason, in order to improve the accuracy of division resistors, Japanese Laid Open Patent Application (JP-P2000-208703A: related art 2) describes a resistor string obtained by connecting a plurality of resistance elements, not the single resistance element, through dividing electrodes. Furthermore, the related art 2 describes a technique of raising the accuracy of divided voltages by manufacturing a pattern by which division electrodes are defined to be a low-resistance element and thereby avoiding variation in resistance in a contact (hereinafter to be referred to as a contact resistance).
On the other hand, in order to reduce display unevenness of a display apparatus such as a liquid crystal display apparatus, high-accuracy gradation voltages are required. Especially, required is a technique of reducing an error between gradation voltages generated by a gradation voltage generating circuit and a gamma curve of desired gradation voltages.
However, in the resistor string in the related arts, differences in resistance among division electrodes that contribute to voltage division (hereinafter to be referred to as a division resistor) are produced due to contact resistances of the contacts into which reference voltages are supplied. For this reason, when the resistor string in the related art is used for the gradation voltage generating circuit, it is difficult to obtain gradation voltages corresponding to a desired gamma curve because an accuracy of the gradation voltages becomes low. Hereinafter, referring to FIGS. 1 and 2, the error of divided voltages (gradation voltages) from the resistor string in the related art will be described.
FIG. 2 is an equivalent circuit of a resistor string 50 shown in FIG. 1. Referring to FIG. 1, in a steady state, a static current I by the reference voltages flows through a path from the wiring 51 to the wiring 52 though the contacts 54-0 to 54-N. For this reason, as shown in FIG. 2, contact resistances rcon0 and rconN due to the contacts 54-0 and 54-N will be formed on a current path. When not taking into consideration the contact resistances rcon0 and rconN, the reference voltages are divided only by the resistors R, and the generated divided voltages V0 to VN are outputted with desired values (ideal values), respectively. However, while the actual divided voltages V1 to VN−1 are affected by voltage drops by the contact resistances rcon0 and rconN, the voltages VG0 and VGN are outputted as the gradation voltages V0 and VN as they are. For this reason, in the resistor string in the related art, relative errors among the divided voltages V0 to VN will become large. Moreover, since there is a possibility that these contact resistances rcon0 to rconN may take different values for every product and for every contact due to a variation at the time of manufacture, the gradation voltage as designed may not be obtained even if a design is made with allowance of the contact resistance, thereby deteriorating a display characteristic. Furthermore, if a selection reference is made severer in order to avoid this problem, the yield of the product may be lowered.
The above-mentioned problems occur similarly in the resistor string described in the related art 2 and in the resistor string made up of a plurality of resistance elements connected together. Especially, in the resistor string formed by connecting the plurality of resistance elements, each contact resistance gives rise to a difference due to manufacturing variation of the contact for connecting the resistance element, and a relative error of each gradation voltage will increase further.